The objective is to define genes and mechanisms which control tumor progression or regression. By using closely related inbred lines of chickens which differ at MHC haplotypes and which differ in ability to regress ASV-induced tumors we can study the function of genes which control the expression of the resistance trait. By using ASV's which are genetically defined we hope to determine the nature of viral induced changes in tumor cells which determine whether a tumor progresses or regresses in a genetically defined host.